Boron-containing compositions for use in clay body e.g. brick manufacture

ABSTRACT

The invention relates to boron-containing compositions for use in clay body, e.g. brick, manufacture. According to the invention clay bodies, such as bricks or tiles, are manufactured by a method comprising mixing into the starting clay a source of B 2 O 2  and a migration inhibitor capable of inhibiting the transport of borate towards the surface of the clay during its drying, forming the clay into the desired shape, drying of firing. Using the method according to the invention clay articles of improved properties may be obtained.

This invention relates to boron-containing compositions for use in claybody, e.g. brick, manufacture. The description below refers principallyto the manufacture of general purpose bricks by way of example. It is tobe appreciated that this is for simplicity only and that the inventionmay be applied to the manufacture of other clay articles, including tilebodies, pipes and pavers.

In brick, and other clay articles, manufacture, wet clay is typicallyformed into the required shape, dried and then fired. It is known toinclude additives in the wet clay.

The properties and make-up of clays used in brick manufacture may varyaccording to the source from which the clay is taken. Soluble salts,particularly sulphates, generally occur naturally in the clay used inbrick manufacture. During the drying cycle of bricks, these solublesalts may migrate to the brick surface, and react during the subsequentfiring step, to form hard deposits of calcium and magnesiumaluminosilicate, which appear as yellowish or whitish discolourations atthe surface. This effect, known as scumming, is noticed when using mostbrick clays and shales.

It is known to include anti-scumming additives in the wet clay. Theseanti-scumming additives work by various mechanisms. For example bariumcarbonate, which controls scum in brick and tile bodies by precipitatingthe naturally occurring soluble sulphates as barium sulphate, isgenerally used in the industry.

AU 507340 proposes the addition of molasses to wet clay, preferably alsowith a reagent which reacts with water-soluble calcium and/or magnesiumsalts to form water-insoluble calcium and/or magnesium salts, to preventscumming brought about by sodium chloride and by water-soluble calciumand/or magnesium salts naturally occurring in the starting clay.

GB-A-2300632 describes using water-soluble liquid syrup carbohydratesderived from the partial hydrolysis of starch to a DE value of less than50 as anti-scumming additives in the manufacture of clay articles, suchas bricks.

Borates are well known as low temperature fluxes, and are used in anumber of industrial applications. In textile fibre glass and glass woolproduction, the use of borates provides benefits such as reduced meltingand liquid temperatures and reduced viscosity. In ceramic glazes theaddition of borate permits the thermal expansion of the glaze to betailored to its particular proposed use.

It has been proposed to add borates, as a source of B₂O₃, to clay to actas fluxing agents. Further borates have been shown previously to reducethe effects of scumming by increasing vitrification at the bricksurface.

Due to their fluxing effect, the addition of borates allows the use ofbroader brick process temperature ranges. In particular they enable therequired properties of the final product to be achieved using lower kilnfiring temperatures, with associated energy savings, than would normallybe required for the clay without fluxing additive. For example, 0.25%and 0.5% by weight B₂O₃ as sodium tetraborate pentahydrate inclusionscan mean that 1050° C. kiln temperatures can be reduced by 25° C. and50° C. respectively. Borates also act as vitrifying agents for claybodies. Vitrification of a brick body increases brick body density,which leads to improving strength and reducing water absorption. Inaddition, it modifies the pore size distribution. These effects arebeneficial in adding value by improving freeze-thaw properties, reducingefflorescence resulting from migration of soluble salts post-firing andallowing firing temperatures to be reduced.

Borate additions thus enable lower grade clays to be used for firedbodies which exhibit properties which can be compared favourably withthose generally attained with higher grade materials. These effects aregenerally more noticeable the higher the borate addition level.

However, although borate addition provides beneficial effects asdescribed above, it may also be associated with unacceptable changes tobrick, or other clay article, appearance, particularly when a watersoluble borate is used as additive. For example, the natural red colourof fired bricks generally deepens with increasing borate additionlevels; the extent of colour change depending on the borate added, brickclay type and drying conditions. Although the change can be seen as acolour enhancement in some cases, it is generally preferred that aborate addition should make minimal visible changes to the bricksurface. Further, during the drying cycle in brick manufacture, moisturemigrates through the brick towards the surface, carrying with it anydissolved borates, which result on firing in surface changes to thebrick through the formation of a glassy layer. Effects are more markedwith soluble, rather than sparingly soluble, borate additives. At highaddition levels, bricks may even stick together.

Previous attempts, which have been made to limit borate migration, haveinvolved the use of borate of lower solubility. Sparingly solubleborates are known to be relatively immobile and surface effects aretherefore reduced, but not avoided entirely. Naturally occurringinsoluble borates are generally unsuitable for use and, while syntheticinsoluble borates are available, they are generally too expensive foruse in clay body, e.g. brick, manufacture. None of the prior art teachesthe prevention of migration of added borate by the use of additionaladditives. As a result of the surface effects encountered, the use ofborate additives in bricks has been limited, and has not been adopted bythe industry.

It has now been found that the advantages associated with the use ofborate as a clay additive may be achieved, without the disadvantagesassociated with surface appearance and properties discussed above, ifthere is used, as additive to clay in the manufacture of clay bodies,such as bricks and tiles, a source of B₂O₃ and a migration inhibitorcapable of inhibiting the transport of borate to the surface of clayduring its drying, prior to firing.

According to one aspect of the. present invention, there is provided theuse, as additive to clay in the manufacture of clay bodies, such asbricks and tiles, of (a) a source of B₂O₃, and (b) a migration inhibitorcapable of inhibiting the transport of borate to the surface of clayduring its drying; components (a) and (b) being added together orseparately.

According to a second aspect of the present invention, there is provideda method of manufacture of clay bodies, such as bricks or tiles, whichmethod comprises:

-   -   mixing into the starting clay, optionally with added water to        achieve the    -   desired plasticity, together or separately, (a) a source of B₂O₃        and (b) a    -   migration inhibitor capable of inhibiting the transport of        borate towards the surface of the clay during its drying;    -   forming the clay into the desired shape;    -   drying the formed clay bodies; and    -   firing the dried formed clay bodies.

The conditions of the clay article manufacture are well known. Thestarting wet clay for forming will generally contain 10 to 30% by weightwater. Typically drying of the formed clay bodies is carried out overseveral hours e.g. 24 to 48 hours, and at temperatures up to 200° C.Firing temperatures are usually in the range 980° C. −1130° C., e.g.1050° C. or could be in the range 1000° C., e.g. 1100° C., to 1200° C.

The source of B₂O₃ used according to the present invention is generallya water soluble borate, e.g. a sodium borate, or boric acid. Mostsuitably a sodium pentaborate, e.g. sodium pentaborate decahydrate, maybe used. In the context of this specification, by the term water solubleborate we mean a borate with a solubility in water greater than 1% byweight at 25° C.

In order to facilitate uniform mixing of the B₂O₃ component with theclay, it is preferable that it be used in solution, in the form of afine powder, or as an aqueous dispersion or slurry. More preferably theborate is used in the form of an aqueous dispersion or slurry in whichthe borate particle size is less than 75, e.g. less than 50, or mostsuitably less than 25 microns. Most preferably the B₂O₃ componentcomprises microfine crystals suspended in a saturated solution.

The borate migration inhibitor (b) must be capable of inhibiting thetransport of borate towards the surface of clay during its drying. Thatis, in a clay in which borate is transported towards the surface of theclay during its drying, that transport is inhibited, including beingprevented, using a borate migration inhibitor according to theinvention.

Suitable materials for use as borate migration inhibitors may be organicpolymers. The materials which may be used as migration inhibitorsinclude starches, gelatin, sugars including sucrose and fructose,lignosulphonates, particularly calcium lignosulphonate, polyvinylalcohol, polyethylene glycol, guar gum, xanthan gum, molasses andwater-soluble (e.g. liquid syrup) carbohydrates derived from the partialhydrolysis of starch to a DE value of less than 50 as described inGB-A-2300632. The economics of brick manufacture are not such as tosupport the use of ingredients which are in any way expensive or have ahigh cost of production. Thus preferred products for use as boratemigration inhibitor (b) according to the present invention are low costmaterials, typically by-products and secondary materials. Preferred suchmaterials are calcium lignosulphonate, molasses and most particularlythe partially hydrolysed starch products.

To facilitate mixing with the clay, the borate migration inhibitor (b)is suitably used as a fine powder, aqueous dispersion or solution.

As mentioned above the actual composition of clays and their propertiesgenerally vary according to their source. High quality clays willtypically require less borate addition than low quality clays. Thus theamount of component (a) to be incorporated into any particular claywill, at least to some extent, depend upon the make-up of the clay. Itwill also depend upon e.g. the extent of fluxing and vitrifying actionrequired. Typically the component (a) content in the final clay article,will be such that it provides 0.1 to 1% B₂O₃ by weight based on theweight of the dried clay article. Generally speaking the content will bein the range of 0.15 to 0.6, e.g. 0.5 %, by weight as B₂O₃.

The mechanism by which the component (b) inhibits the transport of theborate during drying of the clay is not entirely understood. It isbelieved that the inhibitors may form, under the conditions of drying, aphysical barrier, which inhibits the transport of the borates. When anorganic material, the migration inhibitor (b) will be burned off fromthe clay article during firing.

The amount of migration inhibitor (b) used will depend upon the amountof borate additive used. It may also depend upon the clay used sincedifferent clay types will have generally speaking different boratemigration properties, and the presence of materials added to open theclay structure such as sand.

Typically the migration inhibitors are used in an amount such that theirratio, by weight, is from 1 part by weight component (a): 2 parts byweight component (b) to 2 parts component (a): 1 part by weightcomponent (b). Typically the dried clay composition will contain 0.2 to3% by weight of the inhibitor. Generally, up to 1% by weight or morepreferably up to 0.6% inhibitor will be used.

Most preferably the inhibitor used is one which also provides goodanti-scumming properties. Thus the most preferred inhibitor is apartially hydrolysed starch product.

As indicated above components (a) and (b) may be added to the wet clay,prior to forming, separately. However it is generally more convenientthat the ingredients be added together in a single composition, whichmay of course also contain any other additive used.

Additionally, it can be advantageous to apply further migrationinhibitor, generally in solution or suspension, to the surface of theformed bricks prior to drying. This migration inhibitor will augment theeffect of the migration inhibitor within the brick composition.

Accordingly, according to a further aspect of the present invention,there is provided a composition for use as a clay body additive, e.g. inbrick or clay tile manufacture, comprising (a) a source of B₂O₃,preferably a water soluble borate, e.g. a sodium borate, and (b) amigration inhibitor capable of inhibiting the transport of boratetowards the surface of clay during its drying, in particular during itsdrying prior to firing of a clay article containing the composition.

The composition may take the form of a fine powder, aqueous dispersionor solution. Preferably the components (a) and (b) are used in the formof a slurry, i.e. a stable suspension having a fine particle size.

The preferred composition for use according to the present invention areaqueous compositions, typically suspensions, containing components (a)and (b) in the following amounts:

-   -   20 to 55%, preferably 30 to 50%, e.g. 37%, by weight, based on        the total weight of the composition, of component (a); and    -   15 to 45%, preferably 15 to 35%, e.g. 31%, by weight based on        the total weight of the composition, of component (b).

The amount of component (b) in the composition should be such assubstantially to prevent the transport of borate towards the surface ofclay during its drying. As indicated above, this amount may depend uponthe particular clay type being used; some clay types permitting moretransport than others. Generally speaking however the proportions of thetwo components (a) and (b) will be used in the proportions indicatedabove.

Preferred compositions according to the invention are pourable aqueousborate containing suspensions which contain in suspension sodiumpentaborate, as the source of B₂O₃, and a migration inhibitor.Preferably such compositions contain a small proportion of a swellableclay, which assists the pourability of the suspension and maintains itsstability in a manner analogous to that reported in International PatentApplication WO 00/23397. Such suspensions may have a high borate contenton the one hand and on the other hand are pourable making them capableof being easily handable by users. In particular such suspensions may bereadily pumped without clogging of pumping equipment.

Suitable swellable clays include the monmorillonite group of claysparticularly sodium enriched montmorillonite clays and sodiummontmorillonite clays, some of which are known commercially asbentonite, minerals analogous to montmorillonite such as hectorite, andsepiolite.

Suitably, the amount of swellable clay employed in the suspensions asused in the present invention is 0.01-3% by weight, based on the totalweight of the suspension. Generally, the composition would containbetween 0.01 and 1.5% by weight of montmorillonite clays such as IGBclay minerals available from IMV Nevada. When other clays are used, suchas sodium enriched calcium bentonite, e.g. Bentonite MB 300S fromFordimin Company Limited, the amount of swellable clay required may beup to 3% by weight, preferably approximately 2.5% by weight.

A preferred suspension for use according to the present inventioncomprises:

-   -   20 to 55%, e.g. 30 to 55% or 35 to 50%, more preferably 35 to        40%, e.g. 37% by weight sodium pentaborate;    -   15 to 45%, e.g. 15 to 35% or 20 to 30%, and for example 31%, by        weight migration inhibitor, suitably a partially hydrolysed        starch;    -   0.01 to 3% by weight, swellable clay; the remainder being water.

Such suspensions for use according to the present invention may suitablybe prepared by forming sodium pentaborate in suspension, with agitation,in situ by reaction in water of boric acid and sodium tetraborate,typically a hydrated sodium tetraborate, and preferably sodiumtetraborate pentahydrate. Swellable clay should be added before thereaction followed by at least one borate migration inhibitor (b).

According to a preferred embodiment, a fluxing composition forincorporation into clays according to the present invention may beprepared by a method which comprises, in aqueous suspension, allowingboric acid and sodium tetraborate, generally in substantiallystoichiometric amounts for sodium pentaborate, to react under agitation,and before, during or after the reaction, adding to the suspension oradding the suspension to a swellable clay, also dispersed in water,followed by at least one borate migration inhibitor (b).

According to one preferred embodiment, there is provided a method forpreparing a fluxing suspension for use according to the invention, whichmethod comprises dispersing the swellable clay in water, adding boricacid followed by a hydrated sodium tetraborate in substantiallystoichiometric amounts for sodium pentaborate, allowing the mixture toreact while being agitated and then adding at least one borate migrationinhibitor.

The mixing is preferably carried out with a mixing impeller recommendedfor low-viscosity flow-controlled applications such as the Lightin' A310impeller available from Lightnin' Mixers Ltd.

The particle sizes of the borate feed stocks are not critical. They maybe granular, crystalline or finely divided forms. Alternatively, theymay be in the form of wet cakes or slurries.

Preferably the swellable clay is dispersed and hydrated in wateraccording to the manufacturer's recommendations (typically about 10minutes). After this period the mixture may optionally be heated, e.g.up to about 50° C., e.g. to 35° C. to 40° C. Boric acid and sodiumtetraborate are then added and the mixture is allowed to react, withagitation, generally for about one half to one and a half hours, i.e.until the coarse particles of the starting materials are essentially allreacted. At no time should there be complete solution of the borateingredients and sodium pentaborate reaction product. Once the reactionis substantially complete, one or more borate migration inhibitors, inpowder, solution or suspension form, are added to the agitatedsuspension to produce the composition of the invention.

The advantage of this method is that the product comprises a pourablestable suspension wherein there is a high concentration of boron asborate in a finely divided state having particle sizes substantiallyless than 75 microns without the need for a grinding stage. Indeedparticular sizes may be as low as 0.1 to 10 microns.

Typically 0.5 to 5% by weight, preferably 1.0 (e.g. 1.5) to 3.5% byweight, of aqueous composition according to the invention will be addedto the wet clay prior to forming.

The invention is further illustrated with reference to the followingExamples.

EXAMPLE 1

An additive composition was prepared by mixing boric acid (H₃BO₃) withan aqueous suspension of water-soluble liquid syrup carbohydrate derivedfrom the partial hydrolysis of starch to a DE value of less than 50(Activ-7 ex Castle Clays Ltd. and containing approximately 62% by weightsolids) to give a composition comprising 53.3% by weight boric acid andthe balance derived from the hydrated starch suspension. The compositionwas mixed into a clay composition consisting of 58% by weight keupermarl and 42% by weight shale so that the final composition contained0.6% by weight B₂O₃.

Test bricks were produced from this clay mixture by pressing into a die.The bricks were then dried in a electric drying oven at 40° C. for 24hrs followed by 100° C. for 24 hrs.

For comparative purposes, bricks were prepared without the additivecomposition being used.

The dried bricks containing the additive composition were then fired at1010° C., while the comparative bricks were fired at 1050° C., which isthe normal firing temperature for these untreated bricks.

The water absorption and shrinkage properties of the fired bricks werethen measured. The bricks containing borate fired at 1010° C. hadessentially the same properties as the comparative standard bricks firedat 1050° C.

EXAMPLE 2

The boric acid-containing composition described in Example 1 was addedto a keuper marl clay known to show poor freeze-thaw durabilitycharacteristics as described below to give a clay composition containing0.6% by weight B₂O₃ and bricks were prepared and extruded on a plantscale as follows:

Dry clay taken from a stockpile was introduced into the plant on aconveyor belt. The clay was passed through crushing and grinding stagesuntil the typical particle size was 0.3-3 centimetres. Water was added,and at this stage the additive composition described in Example 1 wasintroduced by means of a regulated peristaltic pump. Mixing continued asthe clay went through another milling stage. Finally, additional waterwas added to raise the moisture content to about 15%, suitable forextrusion. The clay mixture was passed to a vacuum extruder, from whichextruded bricks emerged.

The bricks were cut to size and stacked ready for the drying oven. Thebricks were dried for 42 hours, where the bricks underwent a temperatureramp from 20-150° C. At this point, dried bricks were taken and fired ina gas-fired testing kiln.

The frost resistance of the fired bricks significantly improved whencompared to bricks without borate additions. Using a standardfreeze-thaw test rig the number of completed cycles before failures wereseen improved from 30 to 85.

EXAMPLE 3

A poor quality clay was selected that under normal circumstances isblended with a much more expensive higher quality clay in order toachieve adequate fired properties to make it suitable for use in brickmanufacture.

The additive composition described in Example 1 was added to the poorquality clay at a level of 0.6% B₂O₃ using the method described inExample 2.

The formed and dried bricks were then fired 30° C. below the standardfiring temperature of 1050° C.

When compared with standard bricks containing the additional highquality clay, it was found that the fired properties of the bricks wereessentially the same.

EXAMPLE 4 (COMPARATIVE)

Bricks were formed from a clay composition comprising keuper marl (17tonnes) and shale (3 tonnes) prepared as described in Example 2. Aselection of 30 dried bricks was taken and fired in an electric kilnunder laboratory conditions to a peak temperature of between 1000° C.and 1020° C. i.e. ten bricks fired to a peak temperature of 1000° C.,ten to a peak temperature of 1010° C. and ten to a peak temperature of1020° C.

The bricks were then subjected to freeze/thaw testing. Using a standardfreeze/thaw test rig, the number of completed cycles before the bricksfailed was less than 10 in each case.

EXAMPLE 5

Bricks were formed from a clay composition keuper marl (17 tonnes),shale (3 tonnes) and the additive composition of Example 1 (600 kg)prepared as described in Example 2. A selection of 30 dried bricks weretaken and fired in an electric kiln under laboratory conditions asdescribed in Example 4.

The bricks were then subjected to freeze/thaw testing. Using a standardfreeze/thaw test rig, no bricks had failed after 100 completed cycles.

The appearance of the bricks was visually slightly darker than those ofExample 4 but well within quality control limits.

EXAMPLE 6 (COMPARATIVE)

Boric acid and sodium tetraborate pentahydrate (Na₂B₄O₇.5H₂O) were eachadded to laboratory test brick compositions at a constant level of 0.25%B₂O₃. The mixtures were pressed into dies and dried such as to promotemigration to only one side of the brick. Uni-directional migration waspromoted by placing bricks together face-to-face. The pair were thenwrapped in plastic, leaving only the two end faces exposed to the air.They are placed in a drying oven at 100° C. for 24 hours. Thus, dryingoccurs only through the faces exposed to the atmosphere. Migrationtherefore appears at this face only.

The composition of the test bricks (which was formed into 2 test bricks)was as follows:

-   -   82.5 g shale    -   27.5 g sand    -   0.16 g barium carbonate    -   0.56 g sodium tetraborate pentahydrate or 0.48 g boric acid    -   16.48 g water.

The bricks were fired in an electric laboratory kiln with 1° C./mintemperature increase to 1050° C. This temperature was held for 60minutes and then the temperature was reduced at 1.37° C./min to 20° C.

The appearance of the fired bricks showed a very dark, highly glazedsurface when compared with untreated bricks.

EXAMPLE 7

Additive compositions were prepared by adding IGB clay (1 g) to water(39 g) and agitating for 5 minutes. Boric acid (60 g) and sodiumtetraborate pentahydrate (Neobor ex Borax Europe Limited) (46 g) werethen added simultaneously to this mixture. The resulting mixture wasstirred vigorously in a beaker using an electric laboratory stirrer forabout 40 minutes until the mixture had become smooth.

Aqueous suspension of water-soluble liquid syrup carbohydrate (Activ-7as used in Example 1) (106 g) was added as migration inhibitor and theresulting mixture was agitated continuously using a magnetic flea andleft overnight.

There was obtained an aqueous suspension having finely divided particleshaving the following composition (by weight): Sodium pentaborate 37.2%Carbohydrate 26.3% IGB Clay  0.4% Water 36.1%

This suspension (1 g) in water (15.48 g) was agitated for 1 to 2 minutesuntil completed homogenised.

This mixture was then added slowly to a clay composition containingshale (82.5 g), sand (27.5 g) and barium carbonate (0.16 g) being mixedthoroughly in an electric laboratory mixer.

2×55 g of the wet clay mixture obtained were pressed into brick shapesusing a metal die.

The bricks were then dried and fired as described in Example 6.

Compared with Example 6 using no borate migration inhibitor, it wasfound that the additive used here substantially eliminated the adverseeffects of borate migration. Instead of a very dark, glazed surface, thebrick samples had surface appearances similar to untreated bricks i.e.without any sign of glazing.

Similar results were observed, using as borate migration inhibitor,equivalent amounts of polyvinyl alcohol, polyethylene glycol, gelatin,sucrose, fructose, molasses, potato starch, and calcium lignosulphonate.

EXAMPLE 8

Example 7 was repeated but prior to drying, Activ-7 (0.5 g) wasdistributed over the exposed surface of each sample. The samples werethen dried and fired.

The appearance of the bricks after firing was essentially the same assamples that had not had any borate addition.

EXAMPLE 9

Four different additive compositions containing between 25% by weightsodium pentaborate/41% migration inhibitor and 37% by weight sodiumpentaborate/30.5% migration inhibitor (the balance being water) wereadded to a carboniferous coal measure shale known to show poorefflorescence characteristics as described below to give a claycomposition containing between 0.30% and 0.50% by weight B₂O₃ on a dryclay basis.

The migration inhibitor in each case was Activ-7 as used in Example 1containing approximately 62% active ingredient.

The additive compositions were as follows (the balance being water)Sodium Migration Inhibitor Additive Pentaborate (% wt) active ingredient(% wt) 1 25 41 2 31 36 3 33 34 4 37 30.5

Dry clay taken from a stockpile was introduced into the plant on aconveyor belt. It was mixed with asphalt sand in the ratio 75% by weightclay/25% by weight sand. The clay was passed through various crushingand grinding stages until the typical clay particle size was 0.3-3.0 cm.Water is added at a number of points during this process. At this stageone of the additive compositions is added to the mixture by means of acalibrated peristaltic pump. Barium carbonate is also added as standardto this mixture.

Further mixing is carried out in a double-shafted mixer, and then themixture is transferred to a surge hopper. This feeds a vacuum extruderfrom which the extruded clay column emerges. The clay column is cut bywires into brick shapes.

The individual bricks are stacked onto kiln cars ready for drying. Thisprocess happens in a tunnel drying over a period of about 24 hours. Thedried bricks are then fired in a tunnel kiln for about 36 hours to apeak temperature of about 1050° C.

Fired bricks were visually inspected for surface colour and damage. Thewater absorption and shrinkage of selected bricks was also measured.Selected bricks were also subjected to a proprietary efflorescence test,to quantify the efflorescence performance of the bricks.

It was shown that the higher the level of Activ-7 used, the lower thedegree of darkening of surface colour. Also the higher the level ofborate, the lower the amount of efflorescence produced by the brick.Overall it was shown that there is an optimum composition of additivesuch that the level of efflorescence produced by the bricks isminimized, while the prevention of borate migration to the surface ismaximized.

1. Use, as an additive to clay in the manufacture of clay bodies, suchas bricks and tiles, of (a) a source of B₂O₃, and (b) a migrationinhibitor capable of inhibiting the transport of borate to the surfaceof clay during its drying; components (a) and (b) being added togetheror separately.
 2. Use according to claim 1 wherein component (a) is aboric acid or a sodium borate.
 3. Use according to claim 1 whereincomponent (a) is sodium pentaborate.
 4. Use according to claim 1 whereincomponent (b) is a starch, gelatin, sugar, lignosulphonate, polyvinylalcohol, polyethylene glycol, guar gum, xanthan gum, molasses or awater-soluble carbohydrate derived from the partial hydrolysis of starchto a DE value of less than
 50. 5. Use according to claim 1 wherein theratio of components (a) and (b) used is from 1 part by weight component(a): 2 parts by weight component (b) to 2 parts by weight component (a):1 part by weight component (b).
 6. Use according to claim 1 whereincomponent (a) is used in an amount of 0.15 to 0.6% by weight, based onthe dry weight of the clay.
 7. Use according to claim 1 whereincomponent (b) is used in an amount of 0.2 to 3% by weight, based on thedry weight of the clay.
 8. Use according to claim 1 wherein there isused as additive an aqueous composition comprising 20 to 55% by weight,based on the total weight of the composition, of component (a); and 15to 45% by weight, based on the total weight of the composition, ofcomponent (b).
 9. Use according to claim 1 wherein there is used asadditive an aqueous suspension comprising 35 to 40% by weight sodiumpentaborate; 15 to 30% by weight component (b); and 0.01 to 3% byweight, swellable clay; the remainder being water.
 10. A method ofmanufacture of clay bodies, such as bricks or tiles, which methodcomprises: mixing into the starting clay, optionally with added water toachieve the desired plasticity, together or separately, (a) a source ofB₂O₃ and (b) a migration inhibitor capable of inhibiting the transportof borate towards the surface of the clay during its drying; forming theclay into the desired shape; drying the formed clay bodies; and firingthe dried formed clay bodies.
 11. A method according to claim 10 whereincomponent (a) is a boric acid or a sodium borate.
 12. A method accordingto claim 10 wherein component (a) is sodium pentaborate.
 13. A methodaccording to claim 10 wherein component (b) is a starch, gelatin, sugar,lignosulphonate, polyvinyl alcohol, polyethylene glycol, guar gum,xanthan gum, molasses or a water-soluble carbohydrate derived from thepartial hydrolysis of start to a DE value of less than
 50. 14. A methodaccording to claim 10 wherein the ratio of components (a) and (b) usedis from 1 part by weight component (a): 2 parts by weight component (b)to 2 parts by weight component (a): 1 part by weight component (b). 15.A method according to claim 10 wherein component (a) is used in anamount of 0.15 to 0.6% by weight, based on the dry weight of the clay.16. A method according to claim 10 wherein component (b) is used in anamount of 0.2 to 3% by weight, based on the dry weight of the clay. 17.A method according to claim 10 wherein there is used as additive anaqueous composition comprising 20 to 55% by weight, based on the totalweight of the composition, of component (a); and 15 to 45% by weight,based on the total weight of the composition, of component (b).
 18. Amethod according to claim 10 wherein there is used as additive anaqueous suspension comprising 35 to 40% by weight sodium pentaborate; 15to 30% by weight component (b); and 0.01 to 3% by weight, swellableclay; the remainder being water.
 19. A composition for use as anadditive to clay in the manufacture of clay bodies which composition isan aqueous composition comprising 20 to 55% by weight, based on thetotal weight of the composition, of (a) a source of B₂O₃, and 15 to 45%by weight, based on the total weight of the composition, of (b) amigration inhibitor capable of inhibiting the transport of borate to thesurface of clay during its drying.
 20. A composition according to claim19 which is an aqueous suspension comprising 35 to 40% by weight sodiumpentaborate; 15 to 30% by weight component (b); and 0.01 to 3% byweight, swellable clay; the remainder being water.
 21. A compositionaccording to claim 19 wherein component (a) is a boric acid or a sodiumborate.
 22. A composition according to claim 19 wherein component (a) issodium pentaborate.
 23. A composition according to claim 19 whereincomponent (b) is a starch, gelatin, sugar, lignosulphonate, polyvinylalcohol, polyethylene glycol, guar gum, xanthan gum, molasses or awater-soluble carbohydrate derived from the partial hydrolysis of startto a DE value of less than
 50. 24. A composition according to claim 19wherein the ratio of components (a) and (b) used is from 1 part byweight component (a): 2 parts by weight component (b) to 2 parts byweight component (a): 1 part by weight component (b).